Liquid-applied waterproofing membranes are known. In roofing applications they are used as an alternative to prefabricated sheet membranes, offering easier application especially in the case of complex roof geometries and for refurbishment tasks, providing a flexible seamless roof coating which is fully adhered to the substrate.
Liquid-applied waterproofing membranes on roofs have to fulfill demanding requirements. They need to have a low viscosity to be applied as self-levelling coatings and a sufficiently long open time to enable hand application, but still cure fast in order to quickly lose their vulnerability. When fully cured the roofing membrane needs to have durable elasticity and strength in order to protect the building effectively from water ingress in a broad temperature range and under outdoor weathering conditions, such as wind forces, ponding water, frost, strong sunlight irradiation, microbial attack and root penetration.
State-of-the-art liquid-applied waterproofing membranes are often reactive polyurethane compositions, formulated as one-part or as two-part systems, also called single-component or two-component systems, respectively. Two-part systems are more complex to apply, requiring special mixing equipment and proper metering of the two components, since mistakes in mixing quality and/or stoichiometry strongly affect the membrane performance. One-part systems are easy to apply, but prone to curing defects. State-of-the-art one-part systems are based on aliphatic isocyanates and comprise blocked amine hardeners, in particular oxazolidines, to prevent excessive gassing from carbon dioxide formation on curing. They generally contain considerable amounts of solvents to guarantee low viscosity and sufficient shelf life, which is a disadvantage in view of tightening VOC regulations in many countries. Further drawbacks of conventional one-part membranes are related to slow curing and unpleasant odours caused by the emission of the blocking agent, a volatile aldehyde or ketone of strong smell.
To use aromatic instead of aliphatic isocyanates would be interesting from a cost point of view. Moreover aromatic isocyanates afford fast curing properties and high mechanical strength. However, in state-of-the-art one-part waterproofing membranes aromatic isocyanates such as MDI or TDI typically lead to problems with shelf life stability due to premature curing reactions between the aromatic isocyanate groups and the usual blocked amine hardeners. WO 2004/013200 discloses one-part compositions based on aromatic isocyanates comprising special polyaldimines as blocked amine hardeners. These polyaldimines are based on high molecular weight aldol esters such as 2,2-dimethyl-3-lauroyloxypropanal. These compositions have good shelf life stability, but are limited in strength development due to the released high molecular weight aldehyde, which remains in the cured membrane and softens it, acting as a plasticizer. The aldehyde may also migrate to the surface of the the membrane and cause bleeding, thereby limiting the overcoatability of the membrane.
WO 03/059977 discloses one-part compositions based on aromatic isocyanates comprising polyaldimines from lower molecular weight aldehydes. These compositions have good shelf life stability, but are too high in viscosity and therefore unsuitable as liquid-applied waterproofing membranes.